Depends on the cell line. In a lab I was doing MTT some years ago we were doing it always with serum, except one student that had to starve the cells without serum because her cells were hormone responsive and she wanted to see the effect with and without the hormones present. Serum starvation is a common way to remove the hormonal effect i.e. if you have thyroid cells or prostate cancer cells, you do not want to the state of the cells when the specific pathway is already activated by the hormone.